The present invention relates to an improved grid assembly and locking mechanism for positioning a plurality of fuel rods in a nuclear reactor and more particularly to such a combination for use in a gas-cooled gast breeder reactor.
Nuclear fission reactions and the reactors in which they take place are well known in the art. A typical reactor includes a chain-reacting assembly or reactor core made up of nuclear fuel material contained in fuel elements. The fuel material is generally encased in a corrosion resistant heat conductive shell or cladding. The reactor core is made up of a plurality of fuel rods positioned in spaced-apart relation, the reactor core being enclosed within a container through which a reactor coolant is circulated. The reactor coolant is heated during passage through the reactor core and then circulated from the reactor core for extraction of its thermal energy to perform useful work. The coolant is then recycled to the reactor at a reduced temperature to continue the process.
The present invention is particularly concerned with the reactor core for gas-cooled nuclear reactors, especially fast breeder reactors. The core for such reactors consists of a multiplicity of fuel rods which are fastened to a rod holding plate formed with passage holes for the coolant, the fuel rods being connected by means of perforated end caps to a fission gas exhaust system. Such fuel assemblies are commonly referred to as vented fuel assemblies in which fission gases forming within the interiors of the respective fuel rods are transported to a fission gas collection system to prevent excessive pressure from developing within the fuel rod interiors.
The present invention is particularly concerned with a spacer grid assembly for positioning the fuel rods in parallel spaced-apart relation and for resisting substantial disruptive forces which tend to develop within such reactors. For example, the spacer grid assemblies perform a number of functions within the reactor core. Initially, they provide sufficient strength to limit fuel rod bowing and to resist substantial vibration, thermal and hydraulic forces developing within the reactor. The spacer grid assembly also provides sufficient contact with the fuel rods along their length in order to minimize damage to the fuel rods, to accommodate fuel rod swelling and to allow fuel rod insertion or replacement during operation. At the same time, the spacer grid assembly must be designed to minimize neutron absorption and to facilitate the free passage of coolant through the fuel rods in the reactor core.
In a conventional gas-cooled fast reactor spacer grid assembly, a plurality of spacer grids are arranged at spaced intervals along the lengths of the fuel rods. Commonly, each spacer grid is designed for three-point engagement with each fuel rod. Such an arrangement has been found to produce binding between the fuel rods and spacer grids during operation, particularly because of the possibility of differentially swelling in the fuel rods and spacer grids. Since the fuel rods are at a substantially higher temperature than the supporting spacer grid assembly, the fuel rods may experience differential volumetric growth as much as fifty percent greater than the spacer grid assembly.
At the same time, it is necessary to facilitate assembly of the reactor core as well as replacement of its component parts during operation. The need for simplified assembly and replacement techniques may be better understood by considering the large number of fuel rods contained within a typical reactor core.
Accordingly, there has been found to remain a need for an improved spacer grid assembly to assure proper support for the fuel rods within such a reactor core. At the same time, there has also been found to remain a need for an improved locking mechanism for preventing relative rotation of spacer grids arranged along the length of the fuel rods.